In current automobile engine design, it is necessary to meter engine inlet air in order to control engine power output. This is typically accomplished by a butterfly valve located between an inlet air filter and the engine intake manifold. During operation of the engine at idle or low power settings, the engine inlet air undergoes a significant pressure drop while crossing the butterfly valve. At the same time, air accelerates to high velocities while passing through this valve. At times, air speed is in excess of the speed of sound. This air inefficiently diffuses to a lower velocity as it dumps into the large area defined by the engine intake manifold. Significant kinetic energy is created in accelerating this flow. This energy is subsequently depleted in the turbulence which exists during the uncontrolled diffusion into the intake manifold.
Since 1973, most production automobiles have incorporated means for reintroducing a small portion of exhaust back into the engine intake manifold downstream of the combustion air intake. The primary purpose for this is to introduce an "inert" gas (the exhaust gas) into the combustion process to reduce combustion chamber peak temperatures thereby reducing the quantity of nitrous oxides in the exhaust emissions. Exhaust gas recirculation (EGR) reduces engine power due to combustion dilution and reduced inlet air density to the cylinders. Additional disadvantages include the higher susceptibility to engine detonation (knock) because of higher charge air inlet temperatures.
Other systems which have appeared in automobiles utilize shrouds wrapped around hot exhaust manifold components which serve as sources for high temperature air. These hot air supplies are traditionally used for such purposes as the prevention of carburetor ice and to improve fuel atomization in cold conditions to improve vehicle driveability. The disadvantages of this device parallel those of EGR; namely, reduced engine power and higher susceptibility to engine detonation.